Laws of Chemical Combination

Transcript

0:00Introduction

Hello everyone! Okay, one small puzzle before we start. Take one point two grams of magnesium, and burn it in oxygen. When the burning stops, weigh the product, magnesium oxide. Two point zero grams! The product is heavier than the magnesium we started with. So did the fire create mass out of nothing? Wait, that cannot be right, can it? Today we will settle this with three simple laws, the laws of chemical combination. And remember, JEE loves to ask which law a set of data shows, and where these laws fail. So, let's begin.

0:38Law 1: conservation of mass (Lavoisier)

So, let's start with the first law, given by Lavoisier. In any chemical reaction, reactants go in and products come out. Now tell me, does the total mass change? No, not even a little! It stays fixed. This is the law of conservation of mass. In a chemical reaction, the total mass of the reactants equals the total mass of the products. Mass before equals mass after, okay? But notice, the key word here is total. You must count every reactant and every product, not just the ones you can see on the table. Keep this in mind, because our magnesium puzzle hides exactly here. Clear?

1:21Law 2: definite proportions (Proust)

Now the second law, from Proust: the law of definite proportions. It says that a given compound always contains the same elements, in the same proportion by mass, okay? And this is true whatever its source, or the method used to prepare it. So think. Take water from one source, and water made by a completely different method. Will the percentage of hydrogen be different? No! Exactly the same. Water always has eleven point one nine percent hydrogen by mass, and the rest, eighty eight point eight one percent, is oxygen. Any sample, any source, the same numbers. Got it?

2:02Law 3: multiple proportions (Dalton)

Alright, now a doubt. Nitrogen and oxygen form several compounds, and we will see five of them in a moment. Different compositions, from the same two elements! So is Proust's law broken? No. Proust's law talks about one given compound. Each of these compounds has its own fixed composition. And Dalton found a beautiful link between them. This is the law of multiple proportions. When two elements form more than one compound, fix the mass of one element. Then the masses of the other element that combine with this fixed mass are in a simple whole-number ratio. One is to two, two is to three, like that. With me?

2:45Topper's trick: fix one element, compare the other

Now, how do you actually test this in an exam? Here is the topper's trick. Fix the mass of one element: either one gram, or its mass in the simplest formula. Let's try it on the oxides of nitrogen: N two O, N O, N two O three, N O two, and N two O five. Fix nitrogen at twenty eight grams, that is, two nitrogen atoms. Now you tell me, with twenty eight grams of nitrogen, how much oxygen is there in each compound? Look, in N two O, two nitrogen atoms carry one oxygen atom. In N O, two nitrogen atoms carry two oxygen atoms, and so on. So, the masses of oxygen are sixteen, thirty two, forty eight, sixty four and eighty grams. Divide all of them by sixteen, and you get one is to two is to three is to four is to five. Simple whole numbers. Beautiful, isn't it? So remember, when you fix one element, the ratio should come out simple: one is to two, two is to three, one is to two is to three, and so on. If your ratio looks ugly, go back and check which element you fixed, okay?

3:59JEE trap: where the laws fail

Now, the JEE trap. These laws hold for every compound and every reaction, right? No, no, careful! There are exceptions, and examiners love them. First, look at this iron oxide, F e, zero point nine five, O, okay? Only zero point nine five iron atoms for every oxygen atom, not a whole number ratio. Such compounds are called non-stoichiometric. Their composition changes from sample to sample, so the law of definite proportions fails. Second, isotopes. H two O is ordinary water, and D two O is made with deuterium, an isotope of hydrogen. Same elements, but the mass ratio changes. So a compound made from different isotopes does not keep the same proportion by mass. And third, conservation of mass is exact for chemistry, but not for nuclear reactions. There, mass converts into energy. So if a question asks about a nuclear reaction, conservation of mass is the law that fails. Clear?

5:05Example 1: mass of oxygen when magnesium burns

Now, remember our puzzle from the start? Let's solve it properly. One point two grams of magnesium burns in excess oxygen, to give two point zero grams of magnesium oxide. What mass of oxygen reacted? And which law did you use? Pause the video, and try it first. Done? Okay, let's check. Remember the key word, total. Magnesium was not alone, right? Oxygen also reacted. So the mass of magnesium, plus the mass of oxygen, equals the mass of magnesium oxide. So, oxygen equals two point zero minus one point two. Zero point eight grams. And the law? Conservation of mass, of course. So, mystery solved: the fire created nothing. The extra zero point eight grams is just the oxygen that joined the magnesium. See?

6:01Example 2: two oxides of sulphur, multiple proportions

Now a harder one, the JEE type. Two oxides of sulphur contain fifty percent and forty percent sulphur by mass. Show that they obey the law of multiple proportions, and suggest their formulae. Pause and try. Hint: use the topper's trick. Done? Okay, let's check. Each oxide has only sulphur and oxygen. So oxygen is a hundred minus fifty, that is fifty percent, in the first, and a hundred minus forty, that is sixty percent, in the second. Now, many students compare these directly: fifty is to sixty, so five is to six. No, no, careful! This is the JEE trap. In a hundred grams of each oxide, the sulphur is not the same: fifty grams in one, forty grams in the other. The law needs a fixed mass of sulphur. So, fix sulphur at one gram. In the first oxide, oxygen per gram of sulphur is fifty over fifty, that is one point zero. In the second, sixty over forty, that is one point five. One point zero is to one point five: multiply both by two, and you get two is to three. Simple whole numbers, so the law of multiple proportions holds. Perfect! Now the formulae. Take the atomic mass of sulphur as thirty two. So thirty two grams of sulphur, one atom, combines with thirty two grams of oxygen in the first oxide, that is two oxygen atoms, and with forty eight grams in the second, that is three oxygen atoms. So the formulae are S O two, and S O three. See how the trick did all the work?

7:42Quick revision: the three laws of chemical combination

So, quick revision. Conservation of mass: the total mass before a reaction equals the total mass after. Definite proportions: one compound always has one fixed composition by mass. And multiple proportions: fix one element, and expect a simple whole-number ratio for the other. Now go and try the practice problems on this page. You will find them easy, I promise. All the best!